Which statement correctly describes open (dynamic) cohorts?

• A. Closed cohorts have fixed membership after baseline.
• B. Open (dynamic) cohorts require constant membership.
• C. Open cohorts allow new members to join over time or participants to leave; dynamic cohorts track exposure/outcome among changing populations.
• D. Cohort studies cannot be dynamic.

Answer: (C)

Open (dynamic) cohorts are designed so people can enter or leave the study over time. This reflects real-world populations where new individuals may join during follow-up and others may exit due to leaving the study, moving away, or dying. Because membership changes, analyses use person-time and can handle changing exposure status, allowing estimation of incidence and exposure-outcome relationships across a population that isn’t fixed. This flexibility is what sets open cohorts apart from closed ones, where membership is fixed after baseline. Statements that memberships must stay constant or that cohorts cannot be dynamic are not correct, since cohort studies can and often do be dynamic to mirror real-world populations.